Planar optical waveguides with photonic crystal structure

ABSTRACT

Planar optical waveguide comprising a core region and a cladding region comprising a photonic crystal material, said photonic crystal material having a lattice of column elements, wherein at least a number of said column elements are elongated substantially in an axial direction for said core region. The invention also relates to optical devices comprising planar optical waveguides and methods of making waveguides and optical devices.

CROSS-REFERENCE TO RELATED APPLICATION

This application, filed under 35 U.S.C. §363, claims the benefitpursuant to §119(e) of U.S. provisional patent application No.60/364,869 filed on Mar. 15, 2002, the contents of which are hereinincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to planar optical waveguides and devicesthat operate by photonic bandgap effects. The invention provides a newrange of photonic bandgap (PBG) guiding optical waveguides and devicesof a new design, which may be implemented in a number of ways and whichmay be implemented using structures that do need to contain any voids.The optical waveguides and devices covered by the present invention maybe employed for a number of applications, including amplifiers andlasers, coupling devices, and sensors. The invention further provides anumber of methods for fabricating such waveguides and devices.

BACKGROUND OF THE INVENTION

Within the past few years, a significant research interest has beenpointed towards planar optical waveguides and devices that incorporatemicrostructured features—see for example J. D. Joannopoulos, J. N. Winnand R. D. Meade, “Photonic Crystals: Molding the Flow of Light”,Princeton University Press. Princeton, N.J., 1995. Such microstructuresare generally characterized as 1, 2, or 3 dimensional photoniccrystals—depending on the degree of periodicity—and may exhibit photonicbandgap (PBG) effects in 1, 2, or 3 dimensions, respectively. Thepresent invention relates to planar optical waveguides and deviceshaving microstructured features having 2-dimensional (2D) periodicity.

In order to realise photonic bandgap effects in more than one dimension,it is generally believed that materials having relative large indexdifferences must be employed. As known to those skilled in the art, aminimum refractive index contrast of around 1.0 to 2.6 is required forin-plane two-dimensional (2D) PBG effects to take place. Hence,materials such as air (with a refractive index of 1.0) and silica (witha refractive index of 1.45) do not provide sufficient refractive indexcontrast to provide in-plane 2D PBG effects. Indeed, neither doesmaterial systems comprising solely silica and silica co-dopants withrefractive index differences in the range from around 1.40 to 1.50provide sufficient refractive index change for 2D in-plane photonicbandgap effects.

BRIEF SUMMARY OF THE INVENTION

By the present invention it has been realized that low-index contraststructures may, in fact, exhibit useful PBGs in exactly the oppositecase of that taught in the prior art—namely, in the case of low-contraststructures having high-index features disposed in a background materialwith a slightly lower refractive index and wave propagation in thedirection parallel to the microstructured features. In particular, bythe present invention it has been realized that high-index features witha refractive index of around 1.46 disposed in a background material witha refractive index of around 1.45 may provide broadband PBGs that can beutilized for planar optical waveguides and devices. Hence, new planarPBG-based optical waveguides and devices realized purely using silicaand silica incorporating various dopants becomes feasible using thepresent invention, as shall be demonstrated throughout the detaileddescription of the present invention.

For compatibility with conventional, silica-based planar opticaltechnology, it is a disadvantage that prior art PBG waveguides anddevices incorporates high-index contrasts materials, such as for examplevoids and silica, or voids and semiconductors.

The invention provides an improved planar waveguide utilizing thephotonic bandgap technique.

The invention provides functional PBG-based waveguides and devices thatdo not comprise any voids or low-index features at all. In particular,it is an object of the present invention to provide PBG-based waveguidesand functional components that may be realised solely from silica-basedmaterials. Such as to provide such waveguides and devices that may befabricated using index contrasts that are feasible within silicatechnology (for example using Ge, Al, F and/or other dopants that may beincorporated into silica).

It is a further disadvantage of prior art PBG-based optical waveguidesand devices that a large refractive index difference between thecore/cladding features and the background material results in a highsensitivity towards minor structural inaccuracies for certain waveguideor device properties.

The invention further provides PBG waveguides that have small indexcontrast between the constituting materials in order to eliminatedegrading effects such as polarization sensitivity.

Further, it is an objective to provide such a waveguide which may bemanufactured in an improved manner, whereby the necessary resourcesinvolved may be reduced.

The invention also provides such a waveguide which may be manufacturedin a improved manner using readily available processing techniques.

to the invention still further provides such a waveguide which may in anefficient manner be integrated with other optical devices.

The invention also provides a method for manufacturing a planar opticalwaveguide and/or an optical device in an improved and cost-efficientmanner.

These and other advantages are achieved by the invention as explained inthe following.

The invention relates to a planar optical waveguide comprising

-   a core region and-   a cladding region comprising a photonic crystal material,    said photonic crystal material having a lattice of column elements,    wherein at least a number of said column elements are elongated    substantially in an axial direction for said core region.

Hereby a new range of planar waveguides utilizing the photonic bandgaptechnique has been provided. This new range of designs facilitates anumber of advantages, e.g. implementation using new methods and/ormaterials within the field. Thus, a planar waveguide according to theinvention may also be manufactured using readily available processingtechniques. Further, by the invention, integration with other opticaldevices may be performed in a relatively simple manner. It will furtherbe understood that a planar waveguide according to this invention may bemanufactured without using voids, e.g. air voids as is the case with theprior art techniques. Thus, drilling, etching, etc. of holes etc. neednot be performed in order to manufacture a planar waveguide. accordingto the invention.

In a preferred embodiment, said core region may at least partly be inthe form of a defect in said lattice of the photonic crystal material.Hereby a planar optical waveguide using the PBG technique may beprovided in an advantageous manner according to this embodiment.

Advantageously, said core region may comprise a material having a loweffective index of refraction and said cladding region may involve ahigher effective index of refraction. It will thus be understood thatthis embodiment may be implemented using only two different materials orrather two forms of material having different refractive indices.

In a preferred embodiment, said cladding region may comprise abackground material having a first refractive index (n₁), said columnelements may comprise a material having a second refractive index (n₂),and said second refractive index (n₂) may be higher than said firstrefractive index (n₁).

In another preferred embodiment, said cladding region may comprise abackground material having a first refractive index (n₁), said columnelements may comprise a material having a second refractive index (n₂),and said second refractive index (n₂) may be lower than said firstrefractive index (n₁).

In a particular preferred embodiment, an effective refractive ratio forsaid cladding region, e.g. a ratio between said second refractive index(n₂) for said column element(s) and said first refractive index (n₁) forsaid background material, may be defined and said ratio may be less than2.0. Hereby an embodiment has been implemented, whereby useful PBGeffect is provided using a fairly low-index contrast ratio. Thus, planarPBG-based optical waveguides may be realized using a number of materialsthat has not been feasible according to prior art techniques.

Preferably, said effective refractive ratio for said cladding region maybe less than 1.5, in a more preferred form less than 1.3, in a stillmore preferred form less than 1.2 and in a still further preferred formless than 1.1. Hereby a further advantageous embodiment has beenimplemented, whereby useful PBG effect is provided using an even lowerindex contrast ratio.

In a still further preferred form, said effective refractive ratio forsaid cladding region may be less than 1.05, in a more preferred formless than 1.04, in a still more preferred form less than 1.03, and in astill further preferred form less than 1.02. According to thisembodiment, planar optical waveguides having a—compared withconventional techniques—surprisingly low index contrast ratio have beenprovided.

In an advantageous embodiment, said core region may comprise a materialidentical to or similar to a material forming background material ofsaid cladding region. Thus, manufacture may be simplified, e.g. sincefewer materials are needed. The core region may preferably be identicalto the background material or it may comprise background material inmodified form, e.g. in regard to refractive index etc. It will beunderstood that other forms, combinations and medications are possible.

In a further preferred embodiment, said columns elements may comprise amaterial containing impurity elements, e.g. Germanium doped into silicaglass. Hereby, column elements may be manufactured in an advantageousmanner, e.g. by doping, since according to the invention PBG-effect canbe implemented using low-index-contrast ratio. Thus, the change ofrefractive index presented by doping using impurity elements will besatisfactory according to the invention. It is understood that a widevariety of materials, impurities and combinations hereof may be utilizedin accordance with the invention.

In a still further preferred embodiment, said waveguide may compriseglass materials, semiconductor materials, and/or polymer materials.Hereby, a number of materials, the use of which has not been feasibleaccording to prior art techniques, may be utilized for manufacturingplanar PBG-waveguides according to the invention.

Advantageously, said cladding region may comprise a background materialcomprising or consisting of SiO₂ and said background material may have afirst refractive index (n₁), wherein 1.4≦n₁≦1.5, in a more preferredform 1.43≦n₁≦1.47, and in a still more preferred form 1.44≦n₁≦1.45.Hereby, use of a material that has not been employed or studied forconventional planar optical waveguides is presented according to theinvention.

In a further embodiment, said cladding region may comprise a backgroundmaterial comprising or consisting of Si and said background material mayhave a first refractive index (n₁), wherein 2.5≦n₁≦3.0, in a furtherpreferred form 2.6≦n₁≦2.9, and in a still further preferred form2.7≦n₁≦2.8. Hereby, use of a further material that has not been employedor studied for conventional planar optical waveguides is presentedaccording to the invention.

In a still further embodiment, said cladding region may comprise abackground material comprising or consisting of a Group III-V materialand said background material may have a first refractive index (n₁),wherein 3.0≦n₁≦4.5, in a further preferred form 3.3≦n₁≦4.3, and in astill further preferred form 3.7≦n₁≦4.0. Hereby, use of furthermaterials that has not been employed or studied for conventional planaroptical waveguides is presented according to the invention.

Advantageously, said column elements may comprise a material comprisingor consisting of SiO₂ and said material may have a second refractiveindex (n₂), wherein 1.0≦n₂≦1.5, in a preferred form 1.4≦n₂≦1.5, inanother preferred form 1.43≦n₂≦1.47, and in a still further preferredform 1.44≦n₂≦1.45. Hereby, use of a material that has not been employedor studied for conventional planar optical waveguides is presentedaccording to the invention.

In a further embodiment, said column elements may comprise a materialcomprising or consisting of Si and said material may have a secondrefractive index (n₂), wherein 1.0≦n₂≦3.0, in a preferred form2.5≦n₂≦3.0, in another preferred form 2.6≦n₂≦2.9, and in a still furtherpreferred form 2.7≦n₂≦2.9. Hereby, use of a further material that hasnot been employed or studied for conventional planar optical waveguidesis presented according to the invention.

In a still further embodiment, said column elements may comprise amaterial comprising or consisting of a Group III-V material and saidmaterial may have a second refractive index (n₂), wherein 10.0≦n₂≦4.5,in a preferred form 3.0≦n₂≦4.5, in another preferred form 3.3≦n₂≦4.3,and in a still further preferred form 3.7≦n₂≦4.0. Hereby, use of furthermaterials that has not been employed or studied for conventional planaroptical waveguides is presented according to the invention.

Advantageously, said lattice of column elements may comprise a latticeconstant (Λ), a normalized wavelength λ/Λ may be defined by means ofsaid lattice constant (Λ) and a wavelength (λ) for optical wavespropagated by the waveguide and said cladding region may comprise abackground material comprising or consisting of SiO₂, wherein Λ/λ<1.0,in a further preferred form 0.1<Λ/λ<0.8, and in a still furtherpreferred form 0.2Λ/λ<0.5. Hereby, an advantageous embodiment has beenprovided.

In a further embodiment, said lattice of column elements may comprise alattice constant (Λ), a normalized wavelength λ/Λ may be defined bymeans of said lattice constant (Λ) and a wavelength (λ) for opticalwaves propagated by the waveguide and said cladding region may comprisea background material comprising or consisting of Si, wherein Λ/λ<2.0and in a further preferred form Λ/λ<1.5. Hereby, a further advantageousembodiment has been provided.

In a still further embodiment, said lattice of column elements maycomprise a lattice constant (Λ), a normalized wavelength λ/Λ may bedefined by means of said lattice constant (Λ) and a wavelength (λ) foroptical waves propagated by the waveguide and said cladding region maycomprise a background material comprising or consisting of a Group III-Vmaterial, wherein Λ/λ<3.0. Hereby, a still further advantageousembodiment has been provided.

Working on a higher index contrast, it will—with reference to FIG.9.c—be possible to push the mode-splitting point further downwards (withregard to mode index) and at the same time out to larger values of thenormalized wavelength Λ/λ.

Preferably, said cladding region may comprise a background materialhaving a first refractive index (n₁), and an effective guided mode indexmay be lower than said first refractive index (n₁).

Preferably, said column elements may comprise a material having a secondrefractive index (n₂), and an effective guided mode index may be lowerthan said second refractive index (n₂).

The invention also relates to an optical device comprising a planaroptical device. Hereby optical devices involving new combinations ofadvantageous features may be provided e.g. hybrid optical devicescomprising prior art optical devices and planar optical PBG waveguidesaccording to the invention.

Further, the invention relates to an optical device comprising anoptical amplifier and further comprising a planar optical device. Herebyan advantageous design of an optical device may be provided, said designfurther facilitating a number of advantageous features, e.g. includingimproved manufacturing methods, cost efficiency, improved lay-out etc.

The invention also relates to an optical device comprising a laser andfurther comprising a planar optical device. Hereby an advantageousdesign of an optical device comprising a laser construction may beprovided allowing e.g. improved manufacturing methods, cost efficiency,improved lay-out etc.

Still further, the invention relates to an optical device comprising anoptical filter and further comprising a planar optical device. Hereby anadvantageous design of an optical device comprising an optical filterconstruction may be provided.

The invention also relates to an optical device comprising an add-dropmultiplexer and further comprising a planar optical device. Hereby anadvantageous design of an optical device comprising an add-dropmultiplexer construction may be provided.

Further, the invention relates to an optical device comprising anoptical splitter and further comprising a planar optical device. Herebyan advantageous design of an optical device comprising an opticalsplitter construction may be provided.

Still further, the invention relates to an optical device comprising awavelength converter and further comprising a planar optical device.Hereby an advantageous design of an optical device comprising awavelength converter construction may be provided.

The invention also relates to an optical device, said optical devicecomprising means for performing an optical switching, a controllablecoupling or a transferal of optical waves, said optical device furthercomprising a planar optical device. Hereby an optical device forperforming optical switching may be provided in an advantageous mannerutilizing the planar optical waveguide design according to theinvention. A number of advantageous features may be provided in thismanner, e.g. allowing for new manufacturing methods, cost-efficientmanufacturing, new applications etc.

Advantageously, said means for performing an optical switching, acontrollable coupling or a transferal of optical waves may comprise amovable coupling element. Hereby, control of optical switching may beestablished in an efficient manner, directly or indirectly, e.g. throughmeans to move the coupling element, e.g. by heating, change of volume,change of pressure, change of an electromagnetic field etc.

Advantageously, the device may comprise means for actuating said movablecoupling element. Hereby actuation may be provided in a relativelysimple and efficient manner, e.g. using a rod, a string, or other meanssuch as pneumatic, hydraulic etc. means.

In a preferred embodiment, said means for actuating said movablecoupling element may involve the use of mechanical means, meanssensitive to heating and/or cooling, means sensitive to pressure and/ormeans sensitive to electromagnetic fields, voltage, current, strain etc.

In a further preferable embodiment, said device may comprise micro-flowmeans associated with said optical switching, controllable coupling ortransferal of optical waves. Such micro-flow means may for examplecomprise one or more elements of liquids, e.g. liquids for guiding lightand/or for moving separate guiding or switching means. Further, suchliquids, if more than one is used, may not mix, whereby separation ofthe liquids are assured in an efficient manner.

In a particular advantageous embodiment, said micro-flow means mayinvolve utilization of a fluid, in particular two or more fluid elementshaving different refractive indices. Advantageously, such micro-flowmeans may for example comprise different elements of liquids that do notmix and which have different refractive indices. Hereby, modifiedcoupling properties will be achieved by moving said liquids in theoptical device.

Advantageously, said two or more fluid elements comprised in saidmicro-flow system may be separated by mechanical means or preferablysaid two or more fluid elements may be non-mixable fluid elements oressentially non-mixable fluid elements.

The invention also relates to method of making a planar opticalwaveguide, in particular a planar optical waveguide, said methodcomprising steps involving multi-layer depositing and/or processing.Hereby, an optical waveguide or device according to the invention may bemanufactured in an efficient manner, said manner facilitating use of avide variety of suitable materials and/or methods.

Advantageously, said steps may comprise depositing, etching and/orlithographic processes. Such methods are generally know and readilyavailable and allow said manufacturing to be performed in an efficientand cost-effective manner.

The invention also relates to method of making a planar opticalwaveguide, in particular a planar optical waveguide, said methodcomprising steps involving laser induced refractive index changes.Hereby an optical waveguide or device according to the invention may bemanufactured in an efficient manner, e.g. by subjecting a slab to alaser beam, whereby the material in question will change its refractiveindex. An added advantage of this method will be that the lay-out of theelements, e.g. elongated elements, may readily be controlled by movingthe laser apparatus and the slab relatively and/or by controlling theposition of the laser beam.

The invention also relates to method of making a planar opticalwaveguide, in particular a planar optical waveguide, said methodcomprising steps involving self-writing waveguides. Hereby aparticularly advantageous method is provided since the laser beam willcreate a channel having a changed refractive index, said channeldefining an elongated element in e.g. a slab. Hereby, a elongatedelement positioned in the interior of a slab may be manufactured in arelatively simple and accurate manner.

The invention further relates to method of making a planar opticalwaveguide, in particular a planar optical waveguide, said methodcomprising steps involving ion implantation. Hereby a furtheradvantageous method has been provided for creating elements and inparticular elongated elements having a refractive index differing fromthe index of the surroundings, i.e. a method particularly suitable formanufacturing optical waveguides and devices according to the invention.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be explained in further detail below with referenceto the figures of which

FIG. 1 shows in a perspective view a planar photonic bandgap deviceaccording to a prior art technique,

FIG. 2 shows part of such a prior art device seen from above,illustrating the lattice structure,

FIG. 3 shows a structure similar to the one shown in FIG. 2 in aperspective view,

FIG. 4 illustrates typically 2D photonic bandgaps for a prior art 2Dphotonic crystal for use in planar optical circuitry,

FIG. 5 shows a planar photonic bandgap waveguide according to a firstembodiment of the invention in a perspective view,

FIG. 6 shows a waveguide corresponding to the waveguide shown in FIG. 5,but in a modified embodiment,

FIG. 7 shows an embodiment of a planar photonic bandgap waveguidestructure according to the invention,

FIG. 8 shows an alternative embodiment of the embodiment shown in FIG.7,

FIGS. 9 a and b show part of a cross-section of a cladding structure fora planar optical waveguide or planar optical device according to thepresent invention,

FIG. 9 c shows a calculation of allowed modes in a structure asschematically shown in FIG. 9 b,

FIG. 10 shows an optical device according to an embodiment of theinvention, which has been combined with a prior art planar photonicbandgap device,

FIG. 11 shows a further embodiment of an optical device according to theinvention, whereby a coupling or switching of light between waveguidesmay be achieved,

FIG. 12 shows a still further embodiment of an optical device accordingto the invention, whereby a controllable coupling between opticalwaveguides may be achieved and which may be utilized in a vide varietyof applications,

FIG. 13 illustrates a method of manufacturing a planar optical waveguideaccording to the invention by using a multi-layer method,

FIG. 14 illustrates in detail a method for manufacturing a planaroptical waveguide as shown in FIG. 13,

FIG. 15 illustrates an arrangement for manufacturing an optical deviceaccording to the invention by using laser induced refractive indexchanges, and

FIG. 16 illustrates a further method for manufacturing usingself-written waveguides.

DETAILED DESCRIPTION

In the present application it will be distinguished between “refractiveindex” and “effective refractive index”. The refractive index is theconventional refractive index of a homogeneous material. In thisapplication mainly optical wavelengths in the visible to near-infraredregime (wavelengths from approximately 400 nm to 2 μm) are considered.In this wavelength range most relevant materials for waveguideproduction (e.g. silica) may be considered mainly wavelengthindependent, or at least not strongly wavelength dependent. However, fornon-homogeneous materials, such as micro-structures, the effectiverefractive index is very dependent on the morphology of the material.Furthermore the effective refractive index of a micro-structure isstrongly wavelength dependent—much stronger than the refractive index ofany of the materials composing the micro-structure. The procedure ofdetermining the effective refractive index of a given micro-structure ata given wavelength is well-known to those skilled in the art (see e.g.Jouannopoulos et al, “Photonic Crystals”, Princeton University Press,1995 or Broeng et al, Optical Fiber Technology, Vol. 5, pp. 305-330,1999).

Usually a numerical method capable of solving Maxwell's equation on fullvectorial form is required for accurate determination of the effectiverefractive indices of micro-structures. The present invention makes useof employing such a method that has been well-documented in theliterature (see previous Joannopoulos-reference). In the long-wavelengthregime, the effective refractive index is roughly identical to theweighted average of the refractive indices of the constituents of thematerial. For micro-structures, a directly measurable quantity is theso-called filling fraction that is the volume of disposed features in amicro-structure relative to the total volume of a micro-structure. Ofcourse, for waveguides that are invariant in the axial waveguidedirection, the filling fraction may be determined from direct inspectionof the waveguide cross-section.

FIG. 1 shows in a perspective view a planar photonic bandgap devicegenerally designated 1 according to a prior art technique. This planarphotonic bandgap (PBG) device comprises a slab 4 placed between an upper3 and a lower layer 2. These layers 2 and 3 serves to confine opticalwaves to the slab 4 by total internal reflection (TIR). The slab 4according to this prior art technique comprises a 2 dimensional (2D)lattice, e.g. a lattice comprising a number of elements 6 extendingtransversely to the plane of the slab 4 as indicated in FIG. 2 whichshows the slab 4 from above. The elements 6 will exhibit a relativelyhigh index of refraction in relation to the index of refraction of therest of the slab material, e.g. the background material 5, therebycreating a photonic bandgap (PBG) material, i.e. a material whereinlight with a frequency within a certain frequency interval or certainfrequency intervals is not allowed to propagate.

The lattice structure may according to the prior art technique beconfigured in a number of manners, e.g. in a triangular lattice asillustrated, in a quadratic lattice, a honeycomb lattice, a Kagomelattice etc.

According to the prior art technique a line defect is introduced in thislattice structure, for example by omitting a row of elements 6 asillustrated in FIG. 2. Hereby it is achieved that light may propagatethrough the line defect 8. Light with a frequency within the frequencyinterval(s) mentioned above may not escape through the latticestructure, and further the light is confined to the slab 4 caused by thetotal internal reflection by the layers 2 and 3.

A further example of a prior art technique is illustrated in FIG. 3,which shows a slab 4 corresponding to the above described, e.g.comprising a background material 5 with a number of transverselyextending elements or columns 6 arranged in a lattice structure. It willbe understood that an upper and a lower layer (not shown in FIG. 3)facilitating total internal reflection will be arranged. A line defect 8is arranged by omitting a number of elements or columns 6, whereby aregion is defined wherein light may propagate. As illustrated thisregion or line defect 8 may define a waveguide having an angled course,and obviously other forms of courses, e.g. waveguides having curvaturesetc. may be arranged in this manner by designing the line defect(s) 8appropriately.

In the prior art technique the elements or columns 6 have been designedas voids, e.g. cylinders or holes containing air, and have normally beenformed by e.g. drilling or etching holes in the slab 4. Further, by theprior art technique a relatively high index ratio i.e. the ratio betweenthe refractive index for the column elements 6 and the refractive indexfor the background material 5 has normally been desired.

FIG. 4 illustrates a band diagram of a prior art 2D photonic crystalconsisting of air holes in a GaAs background material. The air holeshave a circular size and a diameter, d, of 0.85Λ, where Λ is the pitchof the 2D photonic crystal. As seen from the figure, the 2D photoniccrystal exhibits a complete (i.e. having overlapping Transverse Electric(TE) and Transverse Magnetic (TM) waveguide modes) in-plane bandgaparound a normalized frequency, Λ/λ, of 0.39, where λ is the free-spacewavelength. For complete reflection of light with a wavelength of 1.55μm being incident perpendicular to the axis of air holes, the 2Dphotonic crystal should be designed with Λ of around 0.6 μm and d ofaround 0.5 μm.

A planar photonic bandgap waveguide generally designated 10 according toan embodiment of the invention is illustrated in FIG. 5. As in the casewith the above described prior art, this device comprises a slab 11 withan upper 3 and a lower layer 2 serving to confine light by totalinternal reflection. The slab 11 comprises a lattice of elements orcolumns 12, which are elongated and are arranged substantially in anaxial direction of the slab 11, e.g. in a direction in the plane definedby the slab 11. These elements or columns 12 may be high index elements,i.e. elements having a higher index of refraction than the backgroundmaterial 13 of the slab 11. It will be understood that these elements orcolumns 12 will be parallel or substantially parallel and willfacilitate a photonic bandgap effect. In order to define a waveguide,e.g. a region within which light may propagate, at least one of theelements 12 of the lattice structure is omitted, whereby a line defect16 is created.

As indicated in FIG. 5, the line defect 16 may occupy the total heightof the slab 11, whereby the lattice structure is divided into two parts14 and 15. Hereby the photonic bandgap structure will confine light inlateral directions, but not in transverse direction, thus necessitatingtotal internal reflection by the upper and the lower layers 2 and 3.

However, as illustrated in FIG. 6, the line defect 16 may be restrictedto only part of the slab in the transverse direction, e.g. only one or afew of the elements 16 may be omitted in the transverse direction,whereby confinement will be achieved also in this direction. The upperand the lower layers 2 and 3 will not need to provide internalreflection and may be omitted or may only serve as a protective claddingor for other purposes.

Other embodiments of planar photonic bandgap waveguides according to theinvention are illustrated in FIGS. 7 and 8.

These figures illustrate the elongated elements 12 comprised in awaveguide according to the invention, and it will be understood thatthese elements 12 are surrounded by a suitable background material, e.g.dielectric material.

The elements 12 are arranged in a lattice structure, in FIG. 7illustrated as a quadratic lattice and in FIG. 8 as a triangularlattice, but it will be understood that other forms of lattices may beused as well, such as honeycomb structures, Kagome structures etc. Inboth embodiments a single element has been omitted to create the linedefect or rather the waveguide region 16 within which light maypropagate in the axial direction of the waveguide, confined by thephotonic bandgap effect of the lattice structure. It wil be understoodthat more than one element 12 may be omitted in order to create a linedefect and thus define a core region 16 within which light maypropagate. Thus the size and the form of the core or waveguide region 16may vary in accordance with the number and actual selection ofelement(s) which is/are omitted. Defects may also be formed by placingelements or element groups that are different from the elements 12forming the cladding.

The elements 12 have been illustrated as elements having a quadraticsection but other forms may be used as well, regular forms as well asirregular forms. However, it will be understood that the section of theelements will be substantially uniform along the length of the elements.

According to a further important aspect of the invention the refractiveindices of the background material and the material of the elongatedelements are selected in order to achieve an index contrast, i.e. theratio between these indices, having a significantly lower value thanordinarily used in relation to the prior art technique.

FIG. 9 a shows a part of a cross-section of a cladding structure for aplanar optical waveguide or planar optical device according to thepresent invention. The cross-section is characterized by a number ofperiodically placed features 12 embedded in a background material 13.FIG. 9 b shows schematically a part of a cross-section of a planaroptical waveguide or a planar optical device according to the presentinvention. The planar waveguide or device is characterized by a 2Dphotonic crystal cladding and a central defect 16 formed from a missingfeature. In a preferred embodiment, the features have a refractive indexdifference compared to that of the background material of around 3% orless. Hence, the planar waveguide or device waveguide may be realisedusing silica technology. It is worth noticing that a single claddingfeature 12 may be compared to the core of a conventional planar opticalwaveguide or device, and the novelty of the present invention isemphasised in that the core of the here-disclosed waveguides are formedby placing a number of ‘conventionel cores’ in a periodic manner andleaving out a single (or more) where the light may be guided. Naturally,other arrangements as well as other shapes of the cladding features maybe desired and are also included in the present invention. Furthermore,the core—or defect—may be realised in a number of other manners thanillustrated in this figure—such as for example cores comprising one ormore features of various sizes, shapes and refractive index profiles.

FIG. 9 c shows a calculation of allowed modes in a structure asschematically shown in FIG. 9 b. The structure has a backgroundrefractive index of 1.444 and the cladding features have a refractiveindex of 1.47. The figures show a large number of allowed modes in thecladding structure and a single mode 17 that is confined to the core (ordefect) of the photonic crystal. As seen for the here chosen designparameters (these being mainly the refractive indices of the backgroundmaterial and of the cladding features as well as their size, arrangementand shape), the core mode is only guided from approximately, Λ/λ ofaround 0.25 to 0.50. Hence, for operation at a wavelength of around 1.55μm, Λ should (for this example) be in the range of around 3 μm to 6 μm.The modes are calculated for a non-zero value of propagation constant inthe axial direction of the features, i.e. perpendicular to the periodicplane. Hence, the calculated modes will propagate along the features—asdesired for the defect mode in a planar waveguide configuration.Similarly, this is desired in for example a laser, where reflection attwo separate mirror regions in the longitudinal direction of the planarwaveguide or device, may cause light to travel forth and back and buildup an intense optical field.

It is worth noticing that the effective index of the guided mode isbelow 1.444, i.e. lower than the refractive index of the backgroundmaterial or any other material that the photonic crystal is composed of.This is a unique feature compared to conventional planar opticalwaveguides, where the effective refractive index of one or more guidedmodes are between the refractive indices of the cladding and the corematerial. Hence, the present invention relieves some of the restrictionson core refractive index that characterizes conventional planar opticalwaveguides. In this manner, it becomes for example possible with thepresent invention to utilize new materials for the core—for examplehaving the core formed from a liquid or a polymer, or the core maysimply be formed in pure silica for a low loss planar optical waveguide,where most or all material processing during fabrication is performedaway from the core center. Other possibilities include having a silicacore doped with new materials that lower the refractive index ofsilica—such as materials that are not being employed or have not beenstudied for conventional planar optical waveguides due to their effectof lowering the refractive index. Also other co-dopants, such as forexample F that may presently only be used in small concentration in thecore of a conventional planar optical waveguide or device (such as alaser or amplifier) could be used in larger concentrations in a planaroptical waveguide or device according to the present invention. This mayfor example be beneficial for increased solubility of one or more rareearth elements such as for example Er and/or Yb into the core.

FIG. 10 illustrates a planar optical waveguide according to theinvention generally designated 10, e.g. comprising elongated elements 12in a lattice structure and having a line defect 16, which has beencombined with a prior art planar photonic bandgap device 21, e.g.comprising transversely extending column elements 22, forming a hybridoptical device. Hereby an optical device may be designed involving newcombination of advantageous features.

FIG. 11 illustrates an optical device generally designated 30 accordingto a further embodiment of the invention, comprising a planar opticalwaveguide structure according to the invention. A number of elongatedelements 12 are arranged in a lattice structure, and two or more ofthese elements 12 are omitted, whereby two (or more) waveguides 31 and32 are defined. These may be arranged in parallel or substantiallyparallel as illustrated. Between these waveguides 31 and 32 a couplingelement 33 is arranged, by means of which light propagating along one ofthe waveguides, e.g. 31 may be transferred to the other waveguide 32 (orto one or more other waveguides). The coupling element 33, which servesto “interrupt” the lattice structure of the photonic bandgap structure,whereby light may propagate via the coupling element 33, is preferablymovable as illustrated whereby the coupling point may be controllablymoved. Means to move the coupling element 33 may e.g. be through heatingof closed gas chambers (and thereby change of volume/pressure) at one(or both) sides of a liquid filled section—the coupling element.Alternative approaches may be the application of a material sensitive tolocalized influences such as temperature change, electromagnetic fields,pressure etc. at a specific location along the waveguides.

FIG. 12 illustrates a further optical device generally designated 40according to an embodiment of the invention, comprising a planar opticalwaveguide structure 10 according to the invention. The planar opticalwaveguide structure 10 comprises a number of elongated elements 12arranged in a background material. One 41 (or more) of the elementscomprises a material having a refractive index different from the otherelements whereby a waveguide is provided in the PBG structure 10.Further, another waveguide 42 is arranged in, near or at this opticaldevice. This waveguide 42 may be a conventional waveguide, e.g. anoptical fibre. Further, the optical device comprises a coupling element43, which is movable as illustrated, e.g. connected by a rod, string orthe like to movable means. Alternative means for obtaining the couplingelement 43 may include the application of micro-flow systems containingdifferent elements of liquids that do not mix and which have differentrefractive indices and, therefore, result in modified couplingproperties as the coupling element 43 is moved. For example could an oilfilling surrounded by water sections provide a movable section throughthe use of micro-flow methods shifting the location of the oil section.In a certain position or positions the coupling element may serve toprovide guidance of light from or to the waveguide 42 to or from thewaveguide element 41, whereby the optical device 40 may serve as anoptical switch. The coupling element 43 may be connected to means, whichare movable in response to certain specific circumstances, e.g.temperature, pressure, strain, flow, electric current, voltage, etc.whereby the optical device may serve as a transducer.

FIG. 13 illustrates a planar optical waveguide according to theinvention and in particular a layered construction of such a device.FIG. 13 a illustrates a device 50 having a quadratic lattice structurewhile FIG. 13 b illustrates a device 50′ with a triangular latticestructure. Both comprise a number of layers 51, each of these layerscomprising a planar or essentially planar slab 52, in which a number ofelongated elements 12, e.g. elements having a refractive index differentfrom the material of the slab 52, may be arranged. The waveguide region16 may be provided by omitting one or more of the elements 12 in one ormore of the layers 51, essentially as explained above, or by arrangingone or more element having a suitable refractive index instead of theelements 12. It will be understood that the planar optical waveguidesaccording to the invention may be manufactured by stacking such layerson top of each other.

The individual layers 51 may be manufactured in a number of ways whichwill be illustrated with reference to FIG. 14. FIG. 14 a shows a slab orlayer 52, which may form the basis of a planar optical waveguide. InFIG. 14 b it is illustrated that a number of elongated recesses 53 havebeen made in the layer 52. Such recesses may be made e.g. by etching,machining etc. using templates or the like or without using suchtemplates. These methods are generally available, which will be evidentto a skilled person. In FIG. 14 c it is illustrated that these recesses52 may be filled with a material having a refractive index differingfrom the refractive index of the slab material, i.e. in order to createthe lattice structure of elements 12 according to the invention. Theseelements 12 may be placed in the recesses 53 in a number of ways, e.g.by molding etc. It will be understood that the next step in the processmay be to place a further layer 52 on top of the layer shown in FIG. 14c and then proceed as previously described. The layers may be connectedto each other by means of molding, curing etc.

FIG. 14 d to 14 f illustrates another method of manufacturing layers fora device according to the invention. FIG. 14 d illustrates the basis ofthe process in the form of a layer 52, corresponding to FIG. 14 a. Ontop of this layer elongated members 12 are placed in parallel and inpredefined locations in order to create the lattice structure. Thesemembers may be placed by molding, depositing etc. The next stepillustrated in FIG. 14 f incorporates the introduction of the next layerof background material 54, which may be deposited, molded etc.

It will be understood that such layered construction may be made in anumber of ways, e.g. using depositing, etching and/or lithographicprocesses.

FIG. 15 illustrates a further method of manufacturing an optical deviceaccording to the invention. This method utilizes laser inducedrefractive index changes, whereby the necessary elongated elements 12having differing refractive indices may be made in or on a slab or alayer of dielectric. Such a layer or slab 52 is shown in FIG. 15, and alaser arrangement, e.g. comprising a UV-laser 55 and possibly opticallenses etc. 56, is shown. The laser beam may change the refractive indexof the slab material when appropriately controlled, and by moving thelaser arrangement and the slab 52 in relation to each other, theelongated elements 57 may be formed.

FIG. 16 illustrates a still further method also using a laserarrangement 55, 56 for manufacturing a lattice structure according tothe invention. This arrangement utilizes the self writing effect,whereby waveguides may be self-written. By subjecting a material to alaser beam, e.g. a UV laser beam, the material will change itsrefractive index, and a channel (micro-channel) 58 will be created. Thelaser beam will propagate through this channel 58, and eventually anelongated element having a changed refractive index, i.e. a self-writtenelement, will be created in a linear manner as illustrated by theindication 58 in the slab 52.

Other methods may be used as well, e.g. comprising steps involving ionimplantation etc.

The invention has been described above in general, but it will beunderstood that the waveguide according to the invention may be used inconnection with a wide variety of applications.

It will also be understood that the invention is not limited to theparticular examples described above, but may be designed in a multitudeof varieties within the scope of the invention as specified in theclaims.

1. Planar optical waveguide comprising a core region and a claddingregion comprising a photonic crystal material, said photonic crystalmaterial having a lattice of column elements, wherein at least a numberof said column elements are elongated substantially in an axialdirection for said core region.
 2. Planar optical waveguide according toclaim 1, wherein said core region at least partly is in the form of adefect in said lattice of the photonic crystal material.
 3. Planaroptical waveguide according to claim 1, wherein said core regioncomprises a material having a low effective index of refraction and saidcladding region involves a higher effective index of refraction. 4.Planar optical waveguide according to claim 3, wherein said claddingregion comprises a background material having a first refractive index,said column of elements comprise a material having a second refractiveindex, and said second refractive index is higher than said firstrefractive index.
 5. Planar optical waveguide according to claim 3,wherein said cladding region comprises a background material having afirst refractive index, said column of elements comprise a materialhaving a second refractive index, and said second refractive index islower than said first refractive index.
 6. Planar optical waveguideaccording to claim 4, wherein a ratio between said second refractiveindex for said column element(s) and said first refractive index forsaid background material, is defined and said ratio is less than about2.0.
 7. Planar optical waveguide according to claim 6, wherein saideffective refractive ratio for said cladding region is less than about1.5.
 8. Planar optical waveguide according to claim 6, wherein saideffective refractive ratio for said cladding region is less than about1.05.
 9. Planar optical waveguide according to claim 1, wherein saidcore region comprises a material identical to or similar to a materialforming background material of said cladding region.
 10. Planar opticalwaveguide according to claim 1, wherein said columns elements comprisesa material containing impurity elements.
 11. Planar optical waveguideaccording to claim 1, wherein said waveguide comprises at least one of aglass materials, a semiconductor materials, and a polymer materials. 12.Planar optical waveguide according to claim 1, wherein said claddingregion comprises a background material comprising SiO2 and saidbackground material has a first refractive index (n1), wherein1.4≦n1≦1.5.
 13. Planar optical waveguide according to claim 1, whereinsaid cladding region comprises a background material comprising Si andhaving a first refractive index (n1), wherein 2.5≦n1≦3.0.
 14. Planaroptical waveguide according to claim 1, wherein said cladding regioncomprises a background material comprising a Group III-V material andhaving a first refractive index (n1), wherein 3.0≦n1≦4.5.
 15. Planaroptical waveguide according to claim 1, wherein said column elementscomprise a material comprising of SiO2 and having a second refractiveindex (n2), wherein 1.0≦n2≦1.5.
 16. Planar optical waveguide accordingto claim 1, wherein said column elements comprise a material comprisingSi and having a second refractive index (n2), wherein 1.0≦n2≦3.0. 17.Planar optical waveguide according to claim 1, wherein said columnelements comprise a material comprising of a Group III-V material andhaving a second refractive index (n2), wherein 1.0≦n2≦4.5.
 18. Planaroptical waveguide according to claim 1, wherein said lattice of columnelements comprises a lattice constant (L), a normalized wavelength l/Lis defined by means of said lattice constant (L) and a wavelength (l)for optical waves propagated by the waveguide, and said cladding regioncomprises a background material comprising SiO2, wherein L/l<1.0. 19.Planar optical waveguide according to claim 1, wherein said lattice ofcolumn elements comprises a lattice constant (L), a normalizedwavelength l/L is defined by means of said lattice constant (L) and awavelength (l) for optical waves propagated by the waveguide, and saidcladding region comprises a background material comprising Si whereinL/l<2.0.
 20. Planar optical waveguide according to claim 1, wherein saidlattice of column elements comprises a lattice constant (L), anormalized wavelength l/L is defined by means of said lattice constant(L) and a wavelength (l) for optical waves propagated by the waveguide,and said cladding region comprises a background material comprisingGroup III-V material, wherein L/l<3.0.
 21. Planar optical waveguideaccording to claim 1, wherein said cladding region comprises abackground material having a first refractive index, and wherein aneffective guided mode index is lower than said first refractive index.22. Planar optical waveguide according to claim 1, wherein said columnelements comprise a material having a second refractive index, andwherein an effective guided mode index is lower than said secondrefractive index.
 23. Optical device comprising a planar opticalwaveguide according to claim
 1. 24. Optical device comprising an opticalamplifier further comprising a planar optical waveguide according toclaim
 1. 25. Optical device comprising a laser further comprising aplanar optical waveguide according to claim
 1. 26. Optical devicecomprising an optical filter further comprising a planar opticalwaveguide according to claim
 1. 27. Optical device comprising anadd-drop multiplexer further comprising a planar optical waveguideaccording to claim
 1. 28. Optical device comprising an optical splitterfurther comprising a planar optical waveguide according to claim
 1. 29.Optical device comprising a wavelength converter further comprising aplanar optical waveguide according to claim
 1. 30. Optical devicecomprising means for performing an optical switching, a controllablecoupling, or a transferal of optical waves, said optical device furthercomprising a planar optical device according to claim
 1. 31. Opticaldevice according to claim 30, wherein said means for performing anoptical switching, a controllable coupling, or a transferal of opticalwaves comprise a movable coupling element.
 32. Optical device accordingto claim 31, wherein the device comprises means for actuating saidmovable coupling element.
 33. Optical device according to claim 32,wherein said means for actuating said movable coupling element involvethe use of mechanical means, means sensitive to heating and/or cooling,means sensitive to pressure, and/or means sensitive to electromagneticfields.
 34. Optical device according to claim 30, wherein said devicecomprises micro-flow means associated with said optical switching,controllable coupling, or transferal of optical waves.
 35. Opticaldevice according to claim 34, wherein said micro-flow means involvesutilization of a fluid, comprising two or more fluid elements havingdifferent refractive indices.
 36. Optical device according to claim 35,wherein said two or more fluid elements comprised in said micro-flowsystem are separated by mechanical means or said two or more fluidelements are non-mixable fluid elements or essentially non-mixable fluidelements
 37. Method of making a planar optical waveguide according toclaim 1, comprising multi-layer depositing and/or processing.
 38. Methodaccording to claim 37, further comprising depositing, etching and/orlithographic processes.
 39. Method of making a planar optical waveguideaccording to claim 1, comprising laser induced refractive index changes.40. Method of making a planar optical waveguide according to claim 1,comprising utilizing self-writing waveguides.
 41. Method of making aplanar optical waveguide according to claim 1, comprising ionimplantation.
 42. Planar optical waveguide according to claim 5, whereina ratio between said second refractive index for said column element(s)and said first refractive index for said background material, is definedand said ratio is less than 2.0.
 43. Planar optical waveguide accordingto claim 42, wherein said effective refractive ratio for said claddingregion is less than about 1.5.
 44. Planar optical waveguide according toclaim 42, wherein said effective refractive ratio for said claddingregion is less than about 1.05.